Four U.S. airlines are being investigated for failing to comply with federal aviation regulations.

Federal Aviation Administration officials on Wednesday said an audit of airline maintenance records found three of the four carriers had missed inspection deadlines, while the other failed to submit a required plan.

Two carriers failed to complete inspections of wire bundles for the auxiliary hydraulic pump; the third didn't finish repetitive inspections, according to the FAA.

The results come after a recent string of airline maintenance problems — and on the same day United Airlines grounded dozens of flights.

On Wednesday, United pulled its Boeing 777s from service to test their cargo fire-suppression systems. Last week, American Airlines and Delta Air Lines canceled hundreds of flights to check wiring bundles on some planes.

Also last week, US Airways Group Inc. found problems on some Boeing 757 aircraft after a wing part from another plane fell off during a flight. During subsequent inspections, crews found problems on seven planes and performed minor repairs before returning them to service.

The FAA on Wednesday would not name the four carriers under investigation, but said penalties could be levied. It will be several months before the probes are complete.

Acting FAA Administrator Robert A. Sturgell said the flying public should feel safe despite the maintenance concerns and canceled flights that started with Southwest Airlines Co. last month.

The FAA took the unusual step of ordering the audit of maintenance records at all domestic carriers after reports of missed safety inspections at Southwest. The airline is facing a record $10.2 million fine for continuing to fly dozens of Boeing 737s that hadn't been inspected for cracks in their fuselages.

Industrywide scrutiny of maintenance issues may not the norm, but has become necessary, said Daniel Petree, dean of the College of Business at Embry-Riddle Aeronautical University in Daytona Beach, Fla.

"There are no (recent) examples of poor maintenance or oversight leading to a catastrophe and we don't want to go there," Petree said. "It's appropriate to act before we get there."

The House Transportation and Infrastructure Committee is conducting its own probe of the FAA's oversight of airlines and will report its findings at a hearing Thursday. Among those scheduled to testify are a whistle-blower who first detailed problems at Southwest and the Dallas-based company's chairman and chief executive.

Sturgell said the Southwest case was a "two-way breakdown" involving both the company and the government. But he emphasized that his agency's review of nearly 2,400 airline maintenance records found U.S. carriers in compliance with federal regulations nearly 99 percent of the time.

"The bottom line is ... flying is safer today than at anytime in the past," Sturgell said at a briefing at Washington's Reagan National Airport. "It's no accident or miracle."

Still, the FAA is taking new action both to make it easier for inspectors to raise concerns and to strengthen ethics policies aimed at easing potential conflicts of interests.

The FAA will launch a new reporting system by the end of this month to provide employees an additional way to raise safety concerns they feel are not receiving the necessary attention or response from management, Sturgell said. And by June 30, the agency will start a rule-making process to set a two-year "cooling off" period for former inspectors hired by airlines to match the time that new inspectors hired from industry must wait before they can oversee their former employer.